Crossbar selector switching unit



O c. GILLINGS ETAL- 2,409,122

CROSSBAR SELECTOR SWITCHING UNIT Filgd Dec. 4, 1943 4 Sheets-Sheet- 1 (ace.

I F7 1 IQNVENTORS a DTE CHARLES GILLINGS s3{ 4 LAWRENCE JOHN MURRAY ATTORNEY Oct. s, 1946. v mums HAL 2,409,122

CROSSBAR SELECTGR SWITCHING UNIT Filed Dec. 4, 1945 4 Sheets-Sheet 2 5 i L E GS'U 1 P 10$H m P .F'.E i124" ,41 42 INVENTORS CHARLES GILL! NGS LAWRENCE JOHN MURRAY ATTORNEY CROSSBAR SELECTOR SWITCHING UNIT Filed Dec. 4, 1945 '4 Sheets-Sheet 3 mvsm gns I SHARLES sum 3 LAWRENCE JOHN MURRAY c. GILLINGS ETAL 2,409,122

ATTORNEY 00%. 3, 1946. c, GILLlNss ETAL- 2,409,122

CROSSB ARSELECTOR SWITCHING UNIT Filed Dec. 4, 1943 4 Sheets-Sheet 4 GCG' NE F? g T Wm:

km; ocs v. r YHA 4 G5U1 Fi 4o. 10 IOGSPI 1 IGSP', 1 2 zesn. zossu 1mg T20 16TH ZOOGTH INVENTORSQ CHARLES GILLINGS LAWRENCE JOHN MURRAY ATTO RNEY Patented Oct. 8, 1946 GROSSBAR SELECTOR SWITCHING UNIT Charles Gillings and Lawrence J ohn- Murray, Liverpool, England, assi gnors to Automatic Elece tric Laboratories, Inc, Chicago, 111., a corporation of Delaware Application December 4, 1943, Serial No. 5125888 In Great Britain January 8, 1943 8 Claims. I

The present invention relates to switching equipment for use in telephone and like systems, and is. more particularly concerned with selecting switches of the cross-bar type having sets of contacts arranged to form both rows and columns at right angles to one another.

In our application Serial No. 513,370, filed December I8, 1943, we have described a crossbar switching system in which each of a number of switching stages is controlled by equipment individual to each stage and the object of the present invention is to provide a crossbar selector having improved arrangements for outlet testing whereby the amount of apparatus in the individual control equipment is reduced.

On, say, a ZOO-point crossbar group selector having ten levels and twenty outlets per level there will be provided ten horizontal bar levelselecting relays and twenty vertical bar holding relays whereby connection may be made via the crossbar switch contact springs with any one of the two hundred outlets.

In the above-mentioned application, testing over a group selector level of twenty outgoing links to subsequent selectors was performed by testing directly from the common control group on to the associated outgoing P conductors, there being provided in the control group one or more hunter uniselectors together having ten levels with twenty outlets per level, the particular level involved being selected by'a digit in accordance with the particular group selecting digit dialled.

According to the invention a crossbar selector switching unit has control equipment associated with it which equipment comprises a selector adapted to connect with and test a number of leads corresponding to the number of lines in a group each of said leads being connected to corresponding leads of the group required by elements of the crossbar switching unit itself such elements being selected in accordance with the number of impulses by which the operation of the unit is initially controlled.

Conveniently the control. equipment is individual to the unit which it controls although it willbe appreciated that circumstances may make it desirable fora single unit. to control more than one crossbar selector switching unit.

The invention will be better understood from the following description of one method of carrying it into effect, reference being had to the accompanying drawings comprising Figs. 1-5.

Figs, 1-4 when arranged in the manner indicated in Fig. 40 show circuits of a 200-point receiving switch crossbar group selector and associated common equipment which is assumed to be located in the first numerical group selecting stageof an automatic telephone system such as is described in the above-mentioned application. Fig. 5, which is located on the same sheet as Fig. 4, shows the group selector trunking diagram.

Forconvenience the designations in the present circuits have been made to line up as far as is possible with those ofthe circuits in the prior application and-the group selector shown is there fore assumed to be designated GSUI, while its associated common group is designated GCG,

In the circuits the number of contact units-canried by eachrelay is shown beneath its designation, as for instance (Fig. 3) andt-hese are designated individually in small letters which in this instance-will be bl, 112, etc. A slow-to-release relay is shown with a hatched portion at one end of the rectangle indicative thereof, while a slow-to-operate relay is shown with a crossat one end, such a relay being in some instances slow-to release as well as slowtc-operate.

Asregards the crossbar vertical and horizontal relays, these are each indicated by a hatched rec tangle, the contact units individual thereto being indicated in the same manner'as for ordinary relays. The sets of crossbar switch contacts proper which are closedat the intersection of any given pair of horizontal and vertical bars, for instance the contacts controlled by relays IGSP (Fig. 1) and EGSH (Fig. 2'), are designated and. so. on for each set of contacts involved;

Of the various uni-selectors such as the control group hunter switch which is designated OCS the various sets of wipers and banks are designated OCSI', 0082; etc. The driving mag-- net which is indicated by a hatched square willin this instance be designated OCM, and the magnet interrupter contacts will be designated 0cm. In the case of the digit switch uniselector TDS, since the number of positions required is considerably less than the twenty-five available they are arranged to make two appearances thereon so that this switch will have two home positions I and I2.

Referring now to-Fig; 5, .the group selector comprises two crossbar switching units with their vertical bars in efiect coupled together as shown to form a group selecting unit. More specifically, the coupling will be efiected electrically by the connection or each column of vertically commoned contact springs of the spring sets in one unit with a corresponding column of vertically commoned contact springs n the other unit of the pair.

The selector is of level type and is capable of handling ten connections, and for this purpose the ten incoming links will connect with the horizontal bar relays lGSP-l UGSP. The horizontal bars provided for the purpose of determining the selected one of the ten possible levels are controlled by the relays IGTPIUGTP and the two sets of 20 vertical holding relays lGSI-I2BGSH and lGTH- ZEIGTI-I when operated in pairs such as lGSI-I and lGTI-I or ZGSI-I and ZGTH, and so on, and

in conjunction with one incoming and one level selecting relay, enable the two units of the pair forming the group selector switch proper to function to connect an incoming external link via a free internal link within the selector to. a free one of the twenty outgoing links accessible from aselected level of the selector.

Referring now to the circuits of Figs. 1-4, circuits of the group selector GSUI are shown in the upper parts of Figs. 1 and 2, while circuits of the control group GCG are .shown in the lower parts of these figures and in Figs. 3 and 4.

In the group selector circuit there is shown only the horizontal prepare relay IGSP together with the auxiliary relays IGSR and INF associated with the first out of the ten possible incoming internal links to the selector, which relays, in common with the corresponding relays of the nine other incoming links have access over common leads 3l3l to group GCG. In addition, the vertical hold relays IGSH and IGTH of the first vertical internal link and the horizontal prepare relay IGTP associated with the first of the ten possible outgoing levels are also shown which are operable from group GOG over leads 4043.

The control group GCG comprises a set of relays and two uni-selectors of the 25-point reversedrive electro-magnetically-operated type, the thousands dialled digit being received on switch TDS, whereupon one of the level selecting relays lGTP-l UGTP is operated in accordance with the digit dialled. All the vertical hold relays IGTH- ZUGTH not already in use are thereupon operated, while the hunter switch OCS is set in operation and is caused to test over the selected outgoing level, connection between the contacts of switch OCS and the lines connected to this level being effected via the crossbar contacts at the intersection of the actuated level selecting horizontal bar and the vertical bars. When switch OCS finds a free outlet, the corresponding GSH relay is operated and this maintains a holding circuit for the relevant GTH relay on to the P conductor of the selected outlet when the control group is subsequently released and becomes available for piloting a further call through the same group selector.

When the group selector is taken into use, the calling subscribers loop is extended on to the left-hand incoming negative and positive leads (Fig. l), which leads will relate to the first of theten possible incoming external links to the selector switch, and the terminating crossbar relay IGSP will operate over the following circuit: earth, contacts Igsrd, Igsp5 and lnfll, positive conductor, subscribers loop, negative conductor,

the -level -outlet-per-' contacts lnf3, lgsp l, and |gsr3 to battery via the winding of relay IGSP. This relay at contacts lgsp l and 'lgsp5 extends the calling loop through via common leads 34 and 35 to relay A in the control group GCG and although at these contacts it disconnects its own operating circuit it remains locked via make-first contacts lgspl and contacts lnf5 to lead 36 which is earthed via contacts gsl of relay GS which will have been operated over the incoming P lead from the outlet testing relay in the preceding stage before the loop is extended forward to the group selector.

Relay A in operating at contacts al (Fig. 3) operates relay B in series with rectifier MRD in a conductive direction to battery via resistance YP. Relay B in operating, at contacts bl (Fig. 1)

extends earth over common lead 32 and then via contacts lgspZ to operate relay IGSR in series with resistance YH, at contacts b2 brings up relay BB (Fig. 3), while an alternative locking circuit for relay IGSP (Fig. 1) is completed at contacts b4. At contacts bl (Fig. 3) an energising circuit is completed for relay C on its series-connected windings in series with resistance YQ and switch magnet TDM (non-operative in this condition), while at contacts b8 relay B is locked up independently of the operated contacts al. The group selector relay IGSR in operating at contacts lgsrl extends the earthed lead 32 through to the incoming P lead and this earth extends back to hold the relays in the preceding stage which are connected to the P wire; contacts lgsrl also disconnect the operating circuit for relay GS which commences to release slowly. At contacts lgsr3 it prevents relay IGSP from being re-connected to the incoming negative lead on any subsequent release of this latter relay, while at contacts Igsrd it prevents earth from being connected to the positive lead on the above condition. At contacts lgsr5 the continued holding of relay IGSP is rendered independent of contacts Inf5. In the control group relay BB in operating at contacts bbl (Fig. l) disconnects the relay GS from all the incoming P leads to the selector so as to mark the group selector as busy. At contacts bb2 a dial tone source is connected up over lead DTE to the centre winding of relay A from whence it extends by induction into the outer windings and then on to the speaking leads and back to the calling party. Relay C in operating at contacts 02 (Fig. 3) prepares for the reception of the thousands dialled digit on to magnet TDM of the switch TDS from contacts al of relay A (Fig. 1) which is operated from the calling loop.

The calling subscriber on hearing the dial tone should now proceed to dial the thousands digit of the required number, but in case he should neglect to dial and so hold common equipment GCG unnecessarily, arrangements are made to release this equipment and to return busy tone to the calling subscriber if dialling does not take place within a predetermined period after seizure. This is effected in the following manner by the use of the pair of relays TA and TZ (Fig. 3) in. conjunction with a delay set (not shown) which is common to the exchange and which is arranged to supply earth-pulses bearing a definite time relation to two common leads S and Z. On operation of relay B when the control group GCG is taken in use, contacts b3 (Fig. 3) connect the right-hand winding of relay TA to the S lead and if or when the delay set is in its normal position an earth will be encountered on this lead and relay TA will operate. At contacts tal it locks to earth on to the left-hand winding, at contacts m2 it connects its middle winding and the lower winding of relay TZ to the Z lead, while at contacts me it extends earth on to the delay set start lead ST. After the predetermined period an earth pulse is applied from the delay set to. the Z lead, whereupon relay TZ operates and at contacts 2523 removes the short-circuit from its upper winding over which it looks to the earthed contacts b2 via resistance YJ. At contacts is! and tzfi the initial operating and locking circuits for relay TA are opened but this relay remains held in the circuit including its middle winding and. the lower winding of relay TZ. At contacts tel earth is extended over lead 33 to short-circuit and initiate the release of relay lGSR in the group selector, while at contacts tea (Fig. 2) earth is extended over lead 3'] to bring up relay lNF in the group selector. At the end of the Z pulse, relay TZ remains held and relay TA releases. In the group selector relay tNF in operating at con-- tacts lnfl maintains earth on the incoming P lead and disconnects any circuit therefrom to relay IGSR, while at contacts infi a locking circult is completed for relay iGSR but in this instance this circuit is inefiective owing the short-circuiting over lead At contacts mill and mi l the incoming speaking leads are trans ferred from relay A to relay lNF,-whereupon relay A releases. obtainable tone is, on release of relay GGSR, connected up over common lead NTE to the centre winding of relay lNF from whence it returns to the calling party; relay IGSR in releasing re leases relay EGSP. In the control group, relay A in releasing at contacts at short-circuits and releases relay B, whereupon relays BB and TZ are released, while the TDS switch is stepped offnormal on the release of relay C and is then horned as is described later and the control group GOG is then ready to handle a subsequent call through the group selector.

If the calling party commences to dial within the allowable predetermined period after receipt of dial tone, then on the first release of relay A an energisin circuit is completed for magnet TDM as follows: earth, contacts b3, a! and c2, left-hand low resistance winding of relay C, magnet lDlVl to battery; the winding of relay B is short-circuited during this time, while the righthand winding of relay C is similarly short-circuited in series with resistance YQ. On re-operation of relay A at the end of the first impulse of the dialled train, an energising circuit is re-cornpleted for relay B and relay C now holds operated on both its windings in series with magnet TDM which will not hold in this condition and which thereupon releases to advance its wipers to contacts 2 or it according to'which half of the banks is being used. An operating circuit is now completed for relay N via wiper and bank TDS! operated contacts Z7123 and contacts n2 to battery via resistance YO. This relay in operating, at its contacts 11! opens the holding circuit for relay C and at the same time short-circuits its right? hand winding, at contact 112 prepares for the operation of relay NR and at contacts n3 (Fig. l) disconnects the dial tone circuit.

Succeeding impulses of the train are delivered to magnet TDM and relays B and C hold oper-- ated during this time. It may be mentioned here that the presence of the metal rectifier MRD in the initial energising circuit for relay B serves to prevent the slowing up of the operation of the magnet TDM which would otherwise result from At contacts inffi number-um its the direct connection of a comparatively low value non-inductive resistance such as YP in parallel with its winding; neither magnet TDM nor relay 0 moreover will hold operated in series with relay B.

At the end of the digit, relay A remains oper-- ated and holds relay B, while relay C commences to release slowly. On release of relay C,- contacts ci (Fig. 2 extend earth over operated contacts 114'; and b9 and wiper TDSZ which will be in position 2 or 3 if say digit 1 has been dialled; whereupon the level I crossbar selecting relay IGTP will be operated to actuate the horizontal bar associated with the first one of the ten levels of outlets. Contacts 03 complete an operating circuit for relay NR and this relay in operating at contacts m!- (Flg. 1) further disconnects any circuit to relay GS, and at contacts m2 (Fig. 3) completes a selfinterrupted driving circuit for magnet OCM of switch 003 via bank and wiper OCSl, contacts 7cr2 and lei-r4, magnet interrupter contacts 0cm, magnet OCM to battery, the drivlrig circuit being subsequently maintained by the low resistance winding of relay N. Contacts m3 (Fig. 2) extend earth over lead 43 which is common to all the relays iGTH-NGTH on the group selector, whereupon assuming that none of these relays are already in use, they will allbe operated. Since the level I selecting relay IGTP is already operated, the sets of crossbar contacts on thislevel will be operated and will connect the control group GOG through to the P conductors of all twenty of the outgoing links on level I in the following manner. For instance, as regards relay lGTI-l shown, with this relay operated in addition to relay IGTP, then the set of contacts IGTH IGTP shown in Fig, 2 will be operated and the P lead of the first outgoing link on level I will be connected through over contacts lgsh? and lead 42 tocontact 2 on bank OCS3 of switch OCS. Relay in operating will effect the operation of contacts ZGTH IGTP

(not shown) and contacts 2GTH IGTP will connect the second F lead on level I through via contacts 2gsh2 (not shown) to contact 3 on bank 0083 and so on. By this means the twenty P leads on level I are connected through to contacts 2- 25 inclusive on bank OCS3. On the operation of relay B when the control group was taken into use, contacts 115 and b6 will have connected up the dual testing cut-drive relay KR to wiper 0083, and hence during the rotation of the wipers of switch 008 relay KR will be successively connected to the twenty outgoing Pleads of level ,I in turn. 1 If the P lead connecting with contact 3 of bank OCS3 is already engaged from another group selector, wiper OCS3 will encounter an earth potential and relayKR will not operate. The switch 008 will then proceed to step on to the next position and test the second P lead and if this also proves to be busy the self-interrupted driving operation of switch 0083 will continue until a free outletis encountered which will be marked by an idle marking resistance battery such as that normally extended on to the incoming P leads of the group selector GSUI and relay 7 KR will thereupon operate and at contacts krZ (Fig. 3) will open the magnet OCM driving circuit.

For the purpose of the description it Will be assumed that the first of the twenty outlets on level I is free in which case relay KR will operate as soon as wiper OCS3 reaches position 2. Relay KR in operating at its contacts krl (Fig. 4) extends earth forward over its upper low resistance winding to guard the outlet seized, and at the same time to hold itself operated. At contacts kr2 (Fig. 3) an operating circuit is completed for relay KRR over wiper and bank OCSI to earth via the low resistance winding of relay N. This relay in operating at contacts Icrrl and krrZ (Fig. l) disconnects the operating circuit for relay A, at contacts krr3 (Fig. 4) prepares for the dual testing function of relay KR in case it should be required as will be described later, and at contacts krr4 (Fig. 3) further disconnects the magnet OCM driving circuit. At contacts krr5 (Fig. 4) earth is extended via wiper OCSZ in position 2 and over lead 41 to operate relay IGSH in the group selector, this relay corresponding to the first outlet which has been seized, while contacts krrS (Fig. 4) prepare to maintain relay KR operated independently of the idle marking resistance battery on the selected P lead,

Relay IGSH (Fig. 2) in operating in addition to operating the horizontal selecting relay IGSP initiates the actuation of the set of contacts lGSH and at its own make-first light contacts lgshl (Fig. 4) completes a local locking circuit for relay KR via bank and wiper OCS4 to battery over contacts lam-6 and resistance YHA, while at contacts lgshz it extends its own operating earth through to the P lead of the outgoing link taken into use and also via contacts lgsh3 to maintain relay IGTI-I independently of the contacts nr3.

The calling subscribers loop is therefore now extended through the group selector and out over the first of the twenty outgoing links on level I to the succeeding switch, and relays IGTH, IGSH and IGSR will be held from the earthed lead 4|; in the meantime the control group will be proceeding to free itself in the following manner to be available for further use.

On release of relay A, contacts al (Fig. 3) short-circuit relay B and this relay commences to release slowly. Before it releases an earth will have been returned over the P lead from the succeeding switch so that relays IGSR, (Fig. 1) [GSH and IGTH (Fig. 2) are no longer dependent for their holding on an earth supplied from the control group. On release of relay B, relays BB, KR and KRR release in turn. At contacts M (Fig. l) and b9 (Fig. 2), the group selector prepare relays IGSP and IGTP are released but the pins or their equivalents which are engaged by the vertical bars actuated by relays 381-1 and IGTH remain held. At contacts bb3, R12, and Icrr l (Fig. 3), homing circuits are completed for switches TDS and OCS via the low resistance winding of relay N, the circuit for switch TDS extending over wiper and bank TDSI, contacts b123, interrupter contacts tdm, magnet TDM to battery while the circuit for switch OCS is completed via bank and wiper OCSI, contacts kr2 and MM, interrupter contacts com and magnet OCM to battery. When both these switches reach their home positions, the holding circuit for relay N is opened at wipers and banks TDS! 8 and OCSI and this relay proceeds to release slowly. When it does so, contacts 112 open the holding circuit for relay NR which commences to release slowly and on the release of this relay 5 contacts nrl (Fig. 1) replace the idle marking resistance battery comprising resistance YK and the winding of relay GS on the common test lead 3| of the control group GCG to indicate that this group is now ready to set up any further call 10 through the associated group selector GSU l while at contacts nr3 the earth is removed from common lead 43, whereupon all the vertical hold relays which have been operated therefrom with the exception of relay lGTH, which in this instance remains held from the P lead of the seized outlet, are restored to normal.

In case the OCS switches in two or more control groups should happen to test on the same outgoing link simultaneously. then both of the KR relays will operate over their windings in series to the same idle marking resistance battery and will operate their respective relief relays KRR, while at the same time earth will be extended forward over their upper low resistance windings. In control group GCG relay KRR in operating at contacts krr3 extends a battery potential via resistances YL and YM, and thence via contacts b5 to energize the lower winding in opposition to the upper Winding, similar remarks applying to the KR relay in the other control group.

Now in the case of a single connection from one KR relay to the outgoing P lead the opposing flux produced in the lower winding on operation of relay KRR will not be sufiicient to overcome the holding flux in the low resistance upper winding and the relay will therefore hold. In the case of a dual connection the holding flux produced in each due to the current through their low resistance upper windings will be halved,

while the opposing fluxes due to their lower windings will remain the same. In these circumstances both the KR relays release and cause the associated hunting switches to continue searching. Due to the difference in the speed of any two switches, after a few steps one will reach a free outlet before the other, whereupon the associated KR relay will operate to guard the outlet seized and to disconnect the magnet driving circuit.

In connection with the operation of the GTH relays when the OCS switch is about to hunt, it will be appreciated that in case a vertical internal link within the group selector GSUI is already engaged, then it is necessary to busy the corresponding outlet on all levels of the switch. Assuming that, say, the first internal link within the switch is already engaged, then relays IGTH and IGSH will already be operated when relay NR in the control group operates. Relay NR will operate the other nineteen relays ZGTH-Z DGTH and the switch OCS will then commence to hunt. When it reaches contact 2, since relay IGSH is operated in addition to relay lGTI-I, the test circuit will be broken at contacts lgshZ (Fig. 2)

so that relay KR will be unable to operate and the OCS switch will therefore proceed to advance to the next position.

If all the outlets prove to be busy, the switch OCS will rotate its wipers to contacts 22 when a battery potential via resistance YR will be encountered by wiper OCS3 and relay KR will thereupon operate and will bring in relay KRR. Relay KRR functions as before described with the exception that contacts krr5 (Fig. 4) now extend earth over wiper OCSZ in position 22 and then over lead 31 to operate relay INF over its lefthand winding. Relay INF in operating, at its contacts lnfl maintains a guarding and holding earth on the incoming P lead and at its contacts [M3 and Infll (Fig. 1) connects itself across the incoming speaking conductors to which it locks and at contacts lnj2 (Fig. 1) completes a locking circuit for relay IGSR. At contacts lgsrG (Fig. 3) busy tone is connected up over common lead BTE' to the centre winding of relay INF from whence it returns to the calling party. During this time the common control proceeds to release in normal manner so as to be available for use in setting up further calls in the group selector.

What we claim as new and desire to secure by Letters Patent is:

1. In a switching system, plurality of groups of lines, a numerical switch, means for operating said switch directively to select any one of said groups, a crossbar switch thereupon operated :by said numerical switch to complete connections simultaneously to all of the lines of the selected group, means for testing the lines of the selected group successively over the connections completed to the respective ones of said lines by said crossbar switch, and means operated responsive to said testing means finding a line having a predetermined condition on it fOr causing said crossbar switch to interrupt all of said connections except the one completed to the line having such condition on it.

2. In a switching system, a plurality of groups of lines, means for establishing a predetermined condition upon any one of said lines, a numerical switch, means for operating said switch directively to select any one of said groups, a crossbar switch thereupon operated by said numerical switch to complete a connection to each line in the selected group, and sequence means for testing each line in the selected group in turn over the connection completed to that line by said crossbar switch, thus to determine whether such line has said predetermined condition upon it.

3. In a switching system, a plurality of groups of lines, a plurality of conductors common to said groups, the different ones of said conductors corresponding respectively to the different lines in each group, impulse operated means for selecting any one of said groups, a crossbar switch thereupon operated to connect all of said conductors to the respectively corresponding lines of the selected group, and means including a hunting switch operated to connect with said conductors successively for finding an idle line in the selected group.

4. In a switching system, a crossbar switch having a plurality of groups of lines, a plurality of conductors common to said groups, the difierent ones of said conductors corresponding respectively to the different lines in each of said groups, impulse operated means for selecting any one of said groups and operating said crossbar switch to connect each line of the selected group to the respectively corresponding one of said conductors, testing means, and a switch operated to connect said testin means to the different ones of said conductors at difierent times, thereby to determine whether the difierent ones of the lines in the selected group are idle or busy.

5. In a switching system, a first crossbar switch having a plurality of incoming lines, a second crossbar switch having a plurality of groups of outgoing lines, means for seizing any one of said incoming lines, means thereafter controlled by impulses over the seized line for selecting any one of said groups of outgoing lines and operating said second crossbar switch to complete connections simultaneously from all of the outgoing lines of the selected group to said first crossbar switch, means for testing each of said connections in turn, thereby to find a connection whose associated outgoing line is idle, and means operated responsive to the finding of a connection whose associated outgoing line is idle for operating said first crossbar switch to extend such connection to the seized incoming line.

6. In a switching system, a first crossbar switch having a plurality of incoming lines, a second crossbar switch having a plurality of groups of outgoing lines, means for seizing any one of said incoming lines, means thereafter controlled by impulses over the seized line for selecting any one of said groups of outgoing lines and operating said second crossbar switch to connect with the outgoing lines of the selected group, means for testing the outgoing lines in the selected group successively over the connections completed to those lines by said second switch, thereby to find an idle outgoin line in the selected group, and means operated responsive to the finding of an idle outgoing line to complete a connection from the seized incoming line to said idle outgoing line over said two crossbar switches in series.

'7. In a switching system, a plurality of primary lines, a plurality of groups of secondary lines, control equipment common to said lines, means for seizing said equipment over any one of said primary lines, means in said equipment operated by impulses over said one primary line to select any one of said groups of secondary lines, a crossbar switch thereupon operated by said equipment to complete a connection to each of the secondary lines in the selected group, means for testing each of said connections in turn to determine whether the secondary line associated therewith is idle, and means controlled by said testing means responsive to finding a connection whose associated secondary line is idle for extendin that particular connection to said one primary line, thereby to interconnect said one primary line with said idle secondary line in the selected group.

8. In a switching system as claimed in claim 7, equipment individual to said one primary line, and time controlled means for connecting said individual equipment to said one line and releasing said control equipment in the event that said impulses are not received within a predetermined interval of time after seizure of said control equipment.

CHARLES GILLINGS. LAWRENCE JOHN MURRAY. 

